dark mode and brand yml

[gordonwoodhull]

From posit-dev/brand-yml#68

@Aariq reports

I'm not sure if this belongs here or in the Quarto repo, but it appears like using theme: brand doesn't work for dark mode, only light mode. Perhaps this is just not implemented yet, but I was expecting something like this would work:

format:
  html:
    theme:
      light: brand
      dark:
        - brand
        - dark.scss

where dark.scss only overrides background and foreground colors. However, when I do this, I see that in dark mode the fonts and everything else changes. Even just using light: brand and dark: brand makes it clear that _brand.yml is only applied to light mode.

Tracking here for Quarto 1.7

[coatless]

c.f. discussion: #11487